Mantle source evolution beneath the Cameroon volcanic line: geochemical and geochronological evidences from Fotouni volcanic series, Western Cameroon

We present the first K-Ar geochronological and geochemical data (major and trace elements, as well as Sr, Nd, Pb, and Hf isotopes) on volcanic rocks from the Fotouni lavas in the central part of the Cameroon volcanic line (CVL). Two distinct compositional groups can be discriminated: alkaline and transitional lavas. The alkaline series ranges in composition from basanite and basalt to hawaiite and formed 14 to 3 Ma ago while the transitional series ranges from basalt and trachybasalt to basaltic trachyandesite and formed 47 to 38 Ma ago. All samples display trace element patterns dominated by light rare-earth element enrichments compared to heavy rare-earth elements. The alkaline samples have (87Sr/Sr-86)i ranging from 0.70292 to 0.70340, epsilon(Nd) = 3.45 to 5.98, epsilon(Hf) = 1.95 to 5.63, and relatively radiogenic Pb isotopic compositions: Pb-206/Pb-204 = 19.891-20.592; Pb-207/Pb-204 = 15.639-15.693; Pb-208/Pb-204 = 39.566-40.044. Transitional basalts are characterized by relatively radiogenic Sr and unradiogenic Nd, Hf, and Pb: (epsilon(Nd))i = - 10.82 to + 3.43; epsilon(Hf) = - 14.00 to + 3.13; Pb-206/Pb-204 = 18.0716 to 18.9274; Pb-207/Pb-204 = 15.532 to 15.704; Pb-208/Pb-204 = 37.8141 to 39.4735. Several geochemical characteristics indicate that the transitional lavas have interacted with the continental crust while alkaline lavas did not suffer significant amounts crustal contamination. Both transitional and alkaline lava series can be explained by mixing between two mantle components: an enriched component and a HIMU-like component. These two mantle sources are probably located within the lithospheric mantle and are different from both the Saint Helena and the Mont Cameroon mantle.